With more than 20 recognized species, Monodelphis is the most species-rich genus of living Didelphidae. Recent research on these opossums revealed additional species from Perú and Venezuela, and herein we describe a new species from the montane forests of the eastern slope of the central Andes in Perú. Monodelphis gardneri, new species, is a small taxon with three black dorsal stripes, more similar in external appearance and cranial features to taxa from eastern Brazil, such as M. theresa or M. americana, than to Andean species, like M. osgoodi. Phylogenetic analyses of cytochrome b gene sequences recovered this new species as closer to M. americana from eastern Brazil than to M. theresa (another species in our analyses with three black dorsal stripes). In spite of this external resemblance, the molecular phylogeny indicates that species with dorsal stripes, including M. americana, M. theresa, and the new species, do not form a monophyletic group. Although weakly supported, a relationship between M. americana and the new species suggests a biogeographic connection between these disjunct distributions, i.e., from eastern Brazil and the eastern versant of the Andes.